1. Field of the Invention
The present invention relates to phase change optical recording medium. More concretely, it relates to advanced phase change optical recording medium which have good optical characteristics and good thermal characteristics and of which the absorbance in both the crystalline site and the amorphous site is optimized to reduce cross-erasure and phase jittering in the position of the recorded marks in the medium.
2. Description of the Related Art
Phase change optical recording medium in which information data are recorded and reproduced with optical beams applied thereto have the advantages of large memory capacity, rapid access performance and mobility. In addition, as compared with magneto-optical recording medium, they are further advantageous in that inexpensive CD-compatible drives for them are easy to fabricate since they are based on the same reproduction principle of reflectance change as that for CDs (compact discs), that their memory density is easy to increase since their signal-to-noise ratio is high, and that their data transfer rate is high since one-beam overwriting on them is easy.
An optical disc, which is one example of optical recording medium, is described below.
The operation principle for phase change optical discs is as follows: The recording layer in the disc is melted through exposure to light with the recording power level, and then cooled within a period of time shorter than the crystallization time for the layer to thereby form amorphous marks recorded thereon. The recorded layer is exposed to light with the erasing power level, whereby the layer is heated at a temperature lower than its melting point but not lower than its crystallization point, for which the heating time is kept longer than the crystallization time for the layer. In that manner, the layer is crystallized and the information data recorded thereon are erased. Not depending on the state of the recording layer as to whether the layer is amorphous or crystalline, information data may be written on the layer and the data written thereon may be erased. Therefore, one-beam overwriting on the layer is possible. Reproducing the data written on the layer is based on the difference in the reflectance between the crystalline state and the amorphous state of the layer.
The recording layer may be a thin film of a chalcogenide metal compound, such as GeSbTe. AgInSbTe or InSbTe to which a minor component of Cr, V, N or the like maybe added. Regarding its shape, the disc is typically composed of a polycarbonate substrate with preformatted address and data regions, and a lower dielectric layer, a recording layer, an upper dielectric layer and a reflective layer as laminated on the substrate in that order. An opposite substrate or a label is stuck onto the reflective layer via an adhesive layer therebetween. The dielectric layers and the reflective layer are to prevent the recording layer from being oxidized, to prevent it from being degraded by cumulative overwriting thereon, to modulate the thermal response in recording on the recording layer, and to enhance the optical characteristics of the recording layer in reproducing the data recorded on the layer. In particular, regarding the optical enhancement effect of those layers, the lower dielectric layer exhibits a multi-beam interference effect between the substrate and the recording layer and the upper dielectric layer exhibits a multi-beam interference between the recording layer and the reflective layer, thereby increasing the reflectance change in the recording layer to improve the signal-to-noise ratio of the disc.
In the disc, the reflectance of the amorphous marks (Ra) could be defined freely to be higher or lower than the reflectance of the crystalline area (Rc), depending on the thickness of each layer. Similarly, the absorbance of the amorphous marks (Aa) could also be defined freely to be higher or lower than the absorbance of the crystalline area (Ac).
In recent high-density optical discs with land/groove recording scheme, the recording track-to-track distance is narrowed. In those, one laser ray directed to one track will often spread over the adjacent track regions, and while information data are written or read on one track, other information data written on the adjacent tracks are often erased. The problem of cross-erasure is serious in the discs.
Information data are written as amorphous marks in discs. Therefore, to prevent cross-erasure in discs, the amount of thermal absorption from laser rays in the amorphous marks in adjacent tracks must be lowered. For this, it is desirable that Ac/Aa is at least 1, preferably at least 1.4 or so. Moreover, when Ac/Aa&gt;1, the film temperature at which the film is melted could be similar in the crystalline area and the amorphous area in consideration of the lateral heat for melting, or that is, the overwriting jitter could be reduced (absorbance adjustment).
Regarding the medium constitution that satisfies the requirement of Ac/Aa&gt;1, a structure of substrate/ZnS--SiO2/SiO2/ZnS--SiO2/GeSbTe/ZnS--SiO2/reflective alloy layer was reported orally at Optical Data Storage 98 (Report No. WB3-3), saying that Ac/Aa in the structure is about 1.5.
In the medium reported, however, Rc was defined higher than Ra. Therefore, the medium has some problems mentioned below. In the structure of the medium reported, a thick, heat-insulating dielectric layer is sandwiched between the recording layer and the substrate. In other words, only a so-called slowly-cooling structure could be selected for the medium. Therefore, it is difficult to attain thermal design with Rc higher than Ra for realizing suitable recording/erasure/cross-talk reduction characteristics.
In many phase change optical recording medium heretofore known in the art, Rc is defined higher than Ra. In fabricating those many phase change optical recording medium, after films are formed or after they are laminated, they are subjected to initial crystallization process whereby the recording layer is crystallized throughout the entire surface of the disc. In those, the recording layer not subjected to initial crystallization process is "as deposited", or that is, it is in an amorphous state just as it has been deposited. After having been subjected to initial crystallization, the medium with Rc higher than Ra have high reflectance before being recorded thereon. After recorded, they give signals in the downward direction (that is, in the direction in which their reflectance decreases). Therefore, they are referred to as high-to-low (H-to-L) medium. Their reflectance is increased through initial crystallization, and H-to-L medium have advantages that their address and data regions have high reflectance in the initial stage and provide good servo signal. However, as they have high Rc, their Ac could not be increased so much. Accordingly, H-to-L medium have many problems in that their recording sensitivity is low, the absorbance adjustment indispensable for mark length recording is difficult, and their production costs are high as they indispensably require initialization process.
In particular, when the H-to-L medium with Rc&gt;Ra have no light-absorbing layer except the recording layer, and have a complete reflection-type film structure, they shall naturally have Ac&gt;Aa, and it is impossible to solve absorbance adjustment problem. Some attempt for absorbance adjustment for H-to-L medium with Rc&gt;Ra have been reported. For example, employing an ultra-thin reflective layer as a semi-transparent layer; or a light-absorbing layer is provided between the recording layer and the reflective layer. With those methods, however, the medium modified could have Ac over Aa value by at most 1.2 or so, and they are still unsuitable to higher linear velocity operation for which further absorbance adjustment is needed.
On the other hand, medium of which the film structure with Rc lower than Ra have the advantages of high recording sensitivity and easy absorbance adjustment. In particular, five-layered medium that have a semi-transparent layer of a thin metal film between the substrate and the lower dielectric layer of the four-layered media noted above are suitable to high linear velocity operation, since they can be designed to have Ac&gt;Aa by at least 1.5 by appropriate thickness of the upper and lower interference films and since their crystalline region has high recording sensitivity.
However, with Ac/Aa being higher and with reproduction CNR being higher, Rc of the medium of that type shall be lowered more. Therefore, the medium with Rc&lt;Ra are also problematic, in that their address region becomes difficult to read after initial crystallization process, which is used an the case of the media with Rc&gt;Ra noted above, and the servo signals in the data region are also difficult to read before recording process. In addition, in designating the medium structure with Ac/Aa of around 1.5, the thermal design for realizing the suitable recording/erasure/cross-talk reduction characteristics noted above is difficult, as the latitude in the medium structure is narrow.